Generation and characterization of a knock-in mouse model for Spastic Tetraplegia, Thin Corpus Callosum, and Progressive Microcephaly (SPATCCM)

SLC1A4 (solute carrier family 1 member 4, also referred to as ASCT1, Alanine/Serine/Cysteine/Threonine-preferring Transporter 1) is a sodium-dependent neutral amino acid transporter. It is highly expressed in many tissues, including the brain, where it is expressed primarily on astrocytes and plays key roles in neuronal differentiation and development, maintaining neurotransmitter homeostasis, and N-methyl-D-aspartate (NMDA) neurotransmission, through regulation of L- and D-serine. Mutations in SLC1A4 are associated with the rare autosomal recessive neurodevelopmental disorder spastic tetraplegia, thin corpus callosum, and progressive microcephaly (SPATCCM, OMIM 616657). Psychomotor development and speech are significantly impaired in these patients, and many develop seizures. We generated and characterized a knock-in mouse model for the most common mutant allele, which results in a single amino acid change (p.Glu256Lys, or E256K). Homozygous mutants had increased D-serine uptake in the brain, microcephaly, and thin corpus callosum and cortex layer 1. While p.E256K homozygotes showed some significant differences in exploratory behavior relative to wildtype mice, their performance in assays for motor coordination, endurance, learning, and memory was normal, and they showed no significant differences in long-term potentiation. Taken together, these results indicate that some aspects of SLC1A4 function in brain development are conserved between mice and humans, but the impact of the p.E256K mutation on cognition and motor function is minimal in mice.

Introduction SLC1A4 (solute carrier family 1 member 4, also referred to as Alanine/Serine/Cysteine/Threonine-preferring Transporter 1, ASCT1) is a sodium-dependent neutral amino acid transporter for L-and D-serine, L-alanine, L-cysteine, L-threonine, Lasparagine and L-valine (Foster et al., 2016;Freidman et al., 2020;Kanai et al., 2013). It is highly expressed in skeletal muscle, lung, kidneys, ovary, heart, intestine, and the brain. In the brain, SLC1A4 is expressed by astrocytes and plays a key role in NMDA neurotransmission through regulation of D-serine, as well as generally maintaining neurotransmitter homeostasis (Arriza et al., 1993;Foster et Srour et al., 2015). This autosomal recessive neurodevelopmental disorder has predominantly been reported in patients of Ashkenazi-Jewish descent, with rare cases in other ethnic populations. The most common mutation alters the glutamate at amino acid position 256 to a lysine (p.E256K) and has a carrier frequency of up to 6% in the Ashkenazi-Jewish population (Damseh et al., 2015;Srour et al., 2015). Most patients are unable to achieve independent walking or speech. Some also have seizures, decreased myelination, and/or brain atrophy. A handful of patients have been identi ed with mutations that result in frameshift and are expected to result in SLC1A4 de ciency, but missense mutations are more common. Disease symptoms associated with loss of SLC1A4 are similar to, but more severe than, those observed in patients homozygous for missense mutations (Conroy et al., 2016). In vitro studies indicated that p.E256K mutant SLC1A4 has a higher a nity for L-serine and L-alanine but a lower maximal transport rate than wildtype SLC1A4 (Damseh et al., 2015). Because serine de ciency disorders caused by a defect in L-serine biosynthesis are also characterized by microcephaly, seizures, and psychomotor retardation, L-serine de ciency is generally considered to underlie the neurological phenotypes of SPACTCCM patients. Since L-serine can be metabolized to D-serine by serine racemase and SLC1A4 can transport both entantiomers (Foster et al., 2016) cognitive impairment in SPATCCM patients may re ect an effect on either L-or D-serine transport, or both.
N-methyl-D-aspartate receptors (NMDARs) play important roles in learning and memory in the mammalian brain (reviewed by Orzylowski et al., 2021;Sherwood et al., 2021). NMDARs are activated by binding of synaptic L-glutamate to GluN2/NR2 subunits and D-serine or glycine to co-agonist sites on GluN1/NR1 subunits. D-serine has three-fold higher a nity for the NMDAR co-agonist site than glycine and acts as a physiological regulator of NMDAR signaling. Dysregulation of D-serine levels at synapses is therefore an important potential factor in neuropathologies arising from NMDA receptor hypo-or hyperfunction. SLC1A4 is thought to play a key role in maintaining neurotransmitter homeostasis and NMDA neurotransmission through regulation of D-serine, but it is not clear whether the phenotypes observed in SPATCCM patients re ect an effect of SLC1A4 mutations on D-serine and NMDA signaling or other roles of this transporter in the CNS.
To better understand the pathogenesis of SPATCCM, we generated and characterized a mouse knock-in model for the p.E256K mutation. We demonstrate that p.E256K mutant SLC1A4 has higher a nity for D-serine but a lower maximum rate of uptake, and mice homozygous for this mutation did not show differences in LTP or performance in behavioral tests that assess learning and memory, although they did show increased anxiety behavior. The corpus callosum and cortical layer 1 of the brains of mice homozygous for the p.E256K mutation are thinner than those of wildtype mice, but no signi cant difference in myelination was observed. As comprehension and verbal skills are severely impaired in human patients homozygous for the same mutation, our results suggest there may be species differences in the role/importance of SLC1A4 in the CNS. Slc1a4 em2Tmg mice, referred to herein as Slc1a4 E256K mutants were generated by CRISPR/Cas9 mediated gene editing in mouse embryos using a single guide RNA (sgRNA), CUUCAAUUCCUUCAAUG (Synthego), and a sense HDR single stranded oligodeoxynucleotide (ssODN) repair template: AGCTAGGCCCCGAGGGAGAAGACCTCATCCGATTCTTCAATTCCTTCAATAAGGCCACCATGGTGCTGGTGTCATGGATCATGTGGTG (Integrated DNA Technologies, Inc.). The G > A mutation at c.978 (the rst underlined base) results in p.E256K, while the A > C modi cation at c.986 (the second underlined base) is a silent change intended to insert an NcoI site to facilitate genotyping, but the founder and pups only showed the c.G978A mutation (second underlined base) that results in p.Glu256Lys. The sgRNA and ssODN were resuspended in embryo microinjection buffer ( lter sterilized 5 mM Tris, 0.1 mM EDTA, pH 7.4). The ribonucleoprotein (RNP) mix was prepared by diluting SpCas9 2NLS nuclease (Synthego) and the sgRNA to 4 µM each in Opti-Mem (Gibco) and incubating at room temperature for 10 min prior to adding the ssODN ( nal concentration of 10 µM). The RNP mix was electroporated into 1-cell mouse embryos following published conditions (Troder et al. 2018), after which embryos were incubated in EmbryoMax Advanced KSOM Embryo Medium (Sigma Cat # MR-101-D) with 3 µM Alt-R HDR Enhancer v.1 (Integrated Data Technologies, Inc.), then transferred to pseudopregnant recipient females at the 1-or 2-cell stage. Founders were identi ed by PCR and sequencing, as described below, and mated to C57BL/6J mice to identify heterozygotes, which were intercrossed to generate homozygotes. CRISPR founder mice were identi ed by Sanger sequencing of a PCR product ampli ed from tail DNA (forward primer: GCTTCCCTGCTGAATCTGAC and reverse primer: ACATGGGAAGGTTGCAAGAC). Sequence data was analyzed using Synthego's Inference of CRISPR Edits (ICE) tool. Founders were mated to unmanipulated, wildtype C57BL/6J mice and heterozygotes for the edited allele intercrossed. All the animals used in the studies described here were N1F3-N1F6, descended from one male founder. Age-matched wildtype C57BL/6J mice or wildtype siblings from heterozygous intercrosses were used as controls for all studies.

Mice
Mice were genotyped either by PCR ampli cation and sequencing, as described above, or PCR using the following allelespeci c primers: wildtype forward CCGATTCTTCAATTCCTTCAATG and reverse atgttttctcctcccaccgt (263 bp product), or mutant forward CCGATTCTTCAATTCCTTCAATA and reverse: ttgatgtgagtccaggggtc (492 bp product). Ampli cation reactions used Go-Taq Green Master Mix (Promega). Cycling conditions for the wildtype product were: 95 C for 3 min followed by 33 cycles of 95 C for 30 sec, 61 C for 30 sec and 72 C for 60 sec, followed by 3 min at 72 C. Cycling conditions for the mutant product were: 94 C for 3 min followed by 35 cycles of 94 C for 15 sec, 60 C for 30 sec and 72 C for 45 sec, followed by 72 C for 7 min.

Histology
Brains were xed either at room temperature in 10% formalin for at least 1 week or at 4 C in 4% paraformaldehyde for 5 days prior to standard processing and embedding in para n. Coronal sections were taken at 5 µm, mounted on positively charged slides, and stained with hematoxylin and eosin (H&E) or processed for immunohistochemical (IHC) staining following standard protocols, using an antibody against Neuronal nuclei (NeuN; Millipore Cat#MAB377, RRID:AB_2298772) at 1:100 or myelin basic protein (MBP; Covance Cat# SMI-99, RRID:AB_2314772) at 1:1000, and DAB chromogen (Biolegend Cat# 926507 and 926606) with manual development to ensure that each negative control remained negative and the positive controls developed signal in the appropriate and expected regions. Slides with no primary antibody were used as negative controls. Brains from wildtype mice were used as positive controls. Images were taken on a Zeiss AxioImagerM1 microscope with a Pixielink A623C color camera and morphometry analyzed using ImageJ and the Fiji image-processing package. Brain structures were measured in millimeters after the settings were established on Fiji. Cortex layer 1 thickness was measured in three separate areas evenly spaced across each eld of view. The corpus callosum was measured at the midline. All comparisons between genotypes were made on sections representing similar rostral-caudal regions of the brain.

Behavioral Studies
Behavioral studies were performed on three cohorts of 4-5-month-old Slc1a4 K/K homozygotes and wildtype (Slc1a4 E/E ) controls, all on the C57BL/6J background. The rst cohort of 6 males and 5-6 females of each genotype was assessed for open eld, novel object, Y-maze, balance beam, and grip strength. The second cohort of 4 wildtype and 6 Slc1a4 K/K males and 5 wildtype and 3-4 Slc1a4 K/K females was assessed for open eld, novel object, Barnes maze and grip strength. The third cohort of 7 wildtype and 8 Slc1a4 K/K males and 4 wildtype and 7 Slc1a4 K/K females were subjected to rotarod and grip strength testing. ANY-Maze tracking software (Stoelting Co.) was used for data collection in all studies. Mice were also examined in their home cage for body position, spontaneous activity, tremor, and other general neurobehavioral features.
The novel object tests were performed as described in (Leger et al., 2013) using a 1 day habituation period and white open eld as follows. On day 1, mice were allowed to explore the open eld freely for 5 min (habituation). For the familiarization stage, 24 h later, two identical objects were placed 5 cm from the walls of the open eld and mice were allowed to explore for 10 min. The testing session, where one of the objects was replaced by a new object, occurred 24 h after familiarization and also lasted 10 min. The total number of investigations and time spent investigating each object was recorded for the familiarization and test sessions. An investigation was de ned as the mouse being within 2.5 cm of the object, with its nose pointed towards it. Climbing on the object did not count as an investigation.
The Y-maze test was performed according to standard protocols. Brie y, 16 h before the habituation session, mice were singly housed with fresh bedding. The Y-maze was cleaned with 70% ethanol in between mice to minimize odor cues. During the habituation session, mice were introduced to the maze and allowed to explore freely for 5 min, then returned to their home cage for about an hour before the testing session. For the testing session, bedding was placed in each of the arms as follows: clean bedding in the starting arm (arm C), home-cage bedding in one of the short arms (arm A), and bedding from an age-and sexmatched mouse of the other genotype in the remaining arm (arm B), to act as a novel stimulus. Mice were allowed to freely explore for ve minutes.
The balance beam test measured time to cross a 1 m long, 6 mm wide beam, and consisted of two training sessions and a test session. The beam and escape box were always cleaned with 70% ethanol between mice. During the training sessions, a mouse was placed on a 12 mm thick beam and gently encouraged to go towards an escape box, which held nesting material from the mouse's home cage. If a mouse stopped moving during the run, a gentle tap on the back was given to encourage forward progress. If a mouse refused to go across the beam, the researcher held the mouse by the tail and nudged it toward the escape box. Timing started when the hind legs crossed the start line and ended when the hind legs crossed the nish line. Mice were allowed a brief rest (approx. 15 sec) in the escape box before being repositioned at the start line to repeat the trial. After 3 trials, the mouse was returned to its home cage for a 10-minute break before repeating 3 training runs on a 6 mm wide beam. This entire process was repeated 24 hours after the rst training session. The testing session consisted of three runs across the 6 mm beam 24 h after the second training session.
The Barnes maze test included 1 habituation session, 8 spatial acquisition sessions, and 1 probe session. First, mice were placed on the Barnes maze and allowed to explore freely for 120 sec (habituation). Solid black cues (a triangle, circle, square, and lightning bolt) were placed on the four walls around the maze to provide spatial orientation. At the end of habituation, a clear beaker was placed over the mouse to guide it to the escape hole, where it was kept for 120 sec. The spatial acquisition session took place 24 h after habituation. The target hole and escape tunnel were moved 180 degrees from their original position. Mice were placed under a start cup in the middle of the maze and aversive stimuli (bright lights and 80-decibel hairdryer audio) were used to encourage the mice to nd the target hole as quickly as possible. Mice were released from the start box and allowed to roam freely until they found and entered the target hole, or until 3 min had passed; if the mice did not nd the target hole within 3 min, they were guided to the target hole and allowed to remain there for about 30 sec. Two spatial acquisitions were done per day for four days. For the probe trial, which took place 72 h after the last spatial acquisition session, the escape chamber was closed off. As in the training sessions, the mice were placed under a start box and then allowed to explore the maze freely for 90 sec. Aversive stimuli were present during the probe trial.
Grip Strength was assessed using a Bioseb-GS3 Grip Strength Meter. Mice were held over the grid until they grasped it with only their forepaws, then gently tugged by the tail until they released the grid and maximal peak force recorded. The same process was repeated with all four paws engaging the grid.
For the Rotarod test, mice from the same cage were placed on the apparatus (Maze Engineers) facing forward at a speed of 4 rpm. Once the mice were in placed, the Rotarod accelerated from 4 to 40 rpm over 300 sec. The latency for the mice to fall was recorded. This was done three times a day with at least 15 min in between trials, for three consecutive days.

Statistical Analyses
Unless otherwise indicated, data were analyzed using GraphPad Prism 9. For behavioral studies, differences by genotype were assessed by two-tailed, paired or unpaired T-tests with Welch's correction, except rotarod data, which was assessed using twoway ANOVA for average latency and distance and linear regression for drop speed. Data for males and females of the same genotype were rst assessed separately, then combined if no signi cant difference was observed.

Results
The goals of the studies described here were to determine the effect of a recessive SLC1A4 mutation associated with a human neurodevelopmental disorder on D-serine uptake and to generate and characterize a knock-in mouse model. Several different SLC1A4 mutations have been described in patients with SPATCCM. As the p.E256K allele originally identi ed in the Ashkenazi Jewish population has also been detected in Hispanic and South Asian populations Conroy et al., 2016), we focused on the impact of this mutation.
Mice carrying the c.G978A mutation that alters the glutamate at position 256 to a lysine (p.E256K) were generated by CRISPR/Cas9-mediated gene editing (Fig. 1A-B). Two founders carrying the desired edited allele (c.G978A) were obtained in one experiment: a male carrying the edited allele at a frequency of ~ 28% and a female founder carrying it at ~ 15%. Both transmitted the mutation to their offspring and homozygotes were obtained at Mendelian frequencies from intercrosses between heterozygotes. Homozygotes were viable, fertile, and grossly normal. All mice used in the studies described here descended from the male founder. To assess whether the p.E256K mutation altered SLC1A4 expression, brain protein lysates from 3-month-old C57BL/6J-Slc1a4 K/K and wildtype (Slc1a4 E/E ) controls were subjected to western blotting with an antibody against SLC1A4. Brain lysates from Slc1a4 null mutants, included to verify speci city of the antibody, showed no detectable SLC1A4. No signi cant difference in SLC1A4 levels was observed between Slc1a4 K/K and wildtype brains when normalized to β-tubulin-III (Fig. 1C-D).
Human patients homozygous for the p.E256K mutation have microcephaly, developmental delay, and seizures. Slc1a4 K/K mutant mice did not show any overt phenotypes and were never observed to have seizures, even upon handling, nor were they prone to sudden, unexplained death. We assessed brain and body weight of a cohort of 3-month-old wildtype and Slc1a4 K/K males and females (n = 6-7 per group). There were no differences in body weight ( Fig. 2A). Relative to wildtype mice, brain weight was signi cantly lower in Slc1a4 K/K males, while the difference in females was suggestive but not signi cant (p = 0.06; Fig. 2B). When normalized to body weight, the difference in males (3% lower than wildtype) was no longer signi cant but the difference in females was (7% lower than wildtype; Fig. 2C).
The brains of SPATCCM patients homozygous for the p.E256K mutation show hypomyelination and a thin corpus callosum.
Histopathological analysis of Slc1a4 K/K and control brains at 5 weeks and 5 months of age revealed a signi cant reduction in the thickness of layer 1 of the cortex (Fig. 3A-C) as well as thinning of the corpus callosum at the midline (Fig. 3D-F). IHC for myelin basic protein (MBP) showed a similar pattern and intensity of staining between wildtype and Slc1a4 K/K brains in the corpus callosum (Fig. 3G-H) and other brain regions (data not shown).
It was previously reported that p.E256K mutant SLC1A4 displays higher L-serine and -alanine a nity and lower maximal transport rates, with similar substrate selectivity as wildtype SLC1A4 in a heterologous system (Damseh et al., 2015). As SLC1A4 has more recently been shown to bind D-serine (Foster et al., 2016), we examined the kinetics of SLC1A4 p.E256K transport of D-serine. Expressing p.E256K and wildtype SLC1A4 in frog oocytes, we consistently observed increased a nity of the mutant transporter for D-serine, but a lower maximum velocity (Fig. 4A).Consistent with these data from exogenously expressed SLC1A4 E256K, 300 µm thick acute brain slices from Slc1a4 K/K mice showed increased uptake of 100 nMD-serine relative to slices from Slc1a4 E/E (wildtype) mice (Fig. 4B), suggesting that the mutant protein transports normal physiological concentrations of D-serine at an increased rate.
Altered D-serine homeostasis in the brain could affect NMDAR signaling and have an impact on learning and memory. Consistent with this, human SPATCC patients homozygous for the p.E256K mutation show reduced comprehension and verbal skills. To assess synaptic transmission in the mice, extracellular eld potentials were recorded in the CA1 stratum radiatum of wildtype and Slc1a4 K/K mice ( Fig. 5A-C). Paired stimuli were given 50 ms apart and no differences in amplitude, rise, or decay of the eld EPSPs were measured (Fig. 5A). An input/output relationship was established by plotting the peak amplitude achieved with increasing stimulus strength between (0-400uA) (Fig. 5B). The short-term facilitation induced by paired-pulse stimulation was not statistically different between genotypes (wildtype facilitation ratio, 1.71 +/-.03; Slc1a4 K/K facilitation ratio1.75 +/-.04; Fig. 5C). Synaptic plasticity at this Shaffer-CA1 pyramidal cell synapse was assessed by stimulating in stratum radiatum with a theta burst stimulation protocol to induce long-term potentiation (LTP). The average potentiation in amplitude 30min post-TBS showed no differences between wildtype (1.3 +/-.06) and Slc1a4 K/K (1.28 +/-.06) mice (Fig. 5D).
To evaluate the impact of the p.E256K mutation on behaviors, we subjected Slc1a4 K/K mice to a battery of tests to assess learning, memory, anxiety, locomotion, strength, balance, motor coordination, and endurance. The Barnes maze assesses spatial learning and memory. Consistent with the lack of difference in hippocampal LTP between wildtype and Slc1a4 K/K mice, there were no signi cant differences in time to escape or escape percentage in the Barnes maze test of spatial learning and memory ( Fig. 6A-C). The Y maze was used to assess social exploratory behavior. All animals, regardless of sex or genotype, spent the least amount of time in and had the fewest entries into arm A, containing their home cage bedding, and spent the most time in and had the most entries into arm C, containing fresh bedding (Fig. 6D-F). While there was a non-signi cant trend for wildtype mice, Slc1a4 K/K mice of both sexes did show a signi cant increase in entries into and time spent in arm B (bedding from an unfamiliar cage) than arm A (home cage bedding).
The open eld test assessment of locomotor activity, anxiety, and exploratory behaviors showed that both wildtype and Slc1a4 K/K mice spent signi cantly more time at the edges than in the center (p < 0.002), and there were no signi cant differences between genotypes in total distance traveled or time spent in the center or edges (Fig. 6G-H and data not shown), indicating that the mutants have normal locomotor activity and aversion to bright light and/or open spaces. The novel object test assesses learning and memory as well as exploratory behavior and anxiety. Wildtype and Slc1a4 K/K mice investigated the novel object more frequently but for less time, but the number of times Slc1a4 K/K mice investigated either object was signi cantly lower than controls, and the mutants spent signi cantly less time investigating the novel object compared to wildtype mice (Fig. 6I-J).
As human p.E256K patients show spasticity, we used the grip strength test to measure neuromuscular function based on maximal muscle strength of forelimbs alone and fore-and hind-limbs combined. Data from males and females were combined since they did not show signi cant differences in either genotypic group (p > 0.22). Grip strength for forepaws only and all paws combined showed no signi cantly differences between Slc1a4 K/K and wildtype mice (p > 0.10; Fig. 6K). Consistent with this nding, Slc1a4 K/K mice did not show a signi cant difference in time to cross a 6-mm-wide balance beam (p > 0.60; Fig. 6L).
Wildtype and Slc1a4 K/K mice also showed no signi cant differences in performance on the rotarod (Fig. 6M-O). Wildtype and mutant mice both stayed on the rotarod longer and went further each day of testing (M,N) and they dropped off at a similar rate as rotarod speed increased (shown as percent survival in Fig. 6O).

Discussion
It has generally been assumed that the phenotypes of SPATCCM patients are a result of disrupted L-serine transport, as they overlap with those of serine de ciency disorders (psychomotor retardation, microcephaly and seizures) (Heimer et al., 2015;Damseh et al., 2015;Srour et al., 2015). L-serine plays an essential role in numerous cellular pathways, including protein synthesis, neurotransmission, and synthesis of sphingolipids, phospholipids, L-cysteine, phosphatidyl-L-serine, and D-serine, all of which have important roles in the brain (El-Hattab, 2016; Grant, 2018). The recent identi cation of D-serine as a substrate for transport by SLC1A4 raises the possibility that cognitive impairment in SPATCCM patients re ects disrupted NMDAR signaling [2]. A mouse model of SPATCCM would be a valuable resource for dissecting disease pathogenesis and identifying therapeutic strategies. We therefore used CRISPR/Cas9 gene editing to introduce a c.G978A mutation to alter the glutamate at position 256 to a lysine (p.E256K), replicating a disease-associated mutation found in human SPATCCM patients.
The p.E256K mutation in SLC1A4 was previously shown to reduce maximal rates of L-serine and L-alanine uptake by 50-60%, but to signi cantly increase the transporter's apparent a nity (by 2-3X) for those amino acids (Damseh et al., 2015). Analyzing D-serine uptake in frog oocytes expressing wildtype and p.E256K SLC1A4 and in brain slices from wildtype and Slc1a4 K/K mutant mice, we demonstrate that p.E256K SLC1A4 also has a higher a nity and reduced Vmax for D-serine. While equal amounts of cRNA were transcribed and injected from wild -type and mutant cDNAs that were identical except for the point mutation, a de nitive quantitative comparison of Vmax values cannot be made. Nevertheless the overall kinetic changes seen here with D-serine are consistent with those reported for L-serine by Damseh et al. (2015). This indicates that at low D-serine concentrations, p.E256K likely acts as a gain-of-function mutation, but at high concentrations of D-serine, it would act as a loss-of-function mutation.
Slc1a4 K/K mice had neuroanatomical phenotypes consistent with those observed in SPATCCM patients, including thinning of the corpus callosum and microcephaly. They also showed a signi cant reduction in the thickness of cortical layer 1, but no obvious hypomyelination. Slc1a4 K/K mice were not seizure-prone, but the fact that not all human patients develop seizures suggests this phenotype may be in uenced by genetic or environmental modi ers. SPATCCM patients are typically hypotonic, but strength, locomotor activity, balance, coordination and endurance of in Slc1a4 K/K mice showed no signi cant differences from wildtype, as assessed by the grip strength, open eld, balance beam, and rotarod tests. The results of the novel object recognition test indicated that Slc1a4 K/K mice can distinguish novel and familiar objects, consistent with their performance in the Barnes maze and lack of an LTP phenotype; however, the mutants investigated the novel and familiar objects signi cantly less than wildtype mice, despite not showing any de cits in exploratory behavior in the Y maze. The Y-maze was primarily an olfactory-based paradigm, and also assessed social behavior, while the novel object recognition test was more visual, suggesting there may be differences in how Slc1a4 K/K mutant mice process and interpret different environmental cues. Altered processing of visual information would be consistent with the signi cant reduction in the volume of cortex layer 1 observed in the mutants.
Mutations in SLC1A4 predicted to result in complete loss-of-function are associated with a more severe phenotype in human patients. Mice homozygous for a loss-of-function mutation (Scl1a4 tm1e ) were reported to have decreased hippocampal and striatal volume, lower activity in the open eld, and modestly impaired spatial acquisition and spatial learning reversal in the Morris water maze (Kaplan et al., 2018). We did not see a signi cant difference in spatial acquisition for Slc1a4 K/K mutants in the Barnes maze. Similar to what we observed for Slc1a4 K/K mutants, synaptic plasticity was normal in mice homozygous for the Scl1a4 tm1e allele. Taken together, these results indicate that there are signi cant species differences in the effects of both loss-of-function and gain-of-function SLC1A4 mutations, with generally less severe neurophysiological and behavioral phenotypes seen in Slc1a4 mutant mice. This could suggest that compensatory or redundant mechanisms are present in murine brain that control homeostasis of L-serine, D-serine, and possibly other amino acid substrates of SLC1A4. Studies to understand the mechanism underlying these species-speci c effects could identify novel strategies to treat SPATCCM.  Body and brain weight in Slc1a4 K/K mutants. (A) No signi cant differences were observed in body weight between Slc1a4 K/K and wildtype males or females. (B) Mutant males had signi cantly reduced brain weight; *p=0.01. (C) Mutant females had a signi cantly lower brain weight to body weight ratio than controls; *p=0.02.  The p.E256K mutation alters the kinetics of D-serine uptake. (A) Wildtype or p.E256K mutant human SLC1A4 was expressed in Xenopus oocytes and [3H]-labeled D-serine uptake measured by liquid scintillation spectroscopy. The mutant transporter consistently showed increased a nity for D-serine (Km) but lower maximum velocity (Vmax), indicating saturation at a lower concentration of D-serine. (B) Brain slices from Slc1a4 K/K mice showed higher [3H]-labeled D-serine (100 nM) uptake than Slc1a4 E/E (wildtype) brain slices, consistent with the mutant transporter change in D-serine kinetic parameters.

Figure 5
Sample traces showing representative eld responses in hippocampal slices from wildtype (left, dark traces) or Slc1a4 K/K (right, light traces) mice, evoked by paired stimuli in stratum radiatum (50 ms interpulse interval). (B) Input/output relationship, showing the peak amplitude of the rst EPSP plotted as a function of stimulus strength. (C) The average paired pulse facilitation ratio, taken as the peak amplitude-2/amplitude-1, shows no statistically signi cant differences in this form of short facilitation. (D) Synaptic plasticity was assessed by theta burst stimulation (TBS 1x) in the stratum radiatum. The time series measuring the rst pea amplitude shows a 10-minute baseline, then enhanced synaptic response following the TBS. The inset shows the average long-term potentiation (LTP) taken 30-min post-TBS stimulation for the two genotypes. Figure 6 Neurobehavioral testing was performed to further assess learning and memory, as well as motor coordination, in Slc1a4 K/K mice. (A-C) The Barnes maze assesses spatial learning and memory. There were no signi cant differences in performance between 5-month-old wildtype and Slc1a4 K/K mice. (D-F) The Y-maze assesses social exploratory behavior. Home-cage bedding was placed in arm A, bedding from a foreign cage in arm B, and unused bedding in arm C. All animals, regardless of sex or